Highlight and filter XmlHttpRequests in Web Console
See optimized-out variables in the Debugger
See security warnings in the Network Monitor
See transferred sizes in the Network Monitor
Play/pause all animations in the page
All devtools bugs fixed between Firefox 37 and Firefox 38.
CSS
Support for ruby-position and ruby-align have been added and is available by default (bug 1055676bug 1123917 and bug 1039006).
The :unresolved pseudo-class have been implemented for custom elements (bug 1111633).
The predefined style ethiopic-numeric now uses a space, instead of a dot, as suffix to match a recent change to the spec (bug 1120721).
CSS transitions on generated content (with ::before and ::after) on both an inline and the block that splits them now start as expected by the spec (bug 1110277).
The implementation of CSS Logical Properties made big progress. The following properties are available behind the layout.css.vertical-text.enabled flag (false by default):
Direction-independant equivalents of width and height: block-size and inline-size (bug 1117983).
Direction-independant equivalents of min-width and min-height: min-block-size and min-inline-size (bug 1117983).
Direction-independant equivalents of max-width and max-height: max-block-size and max-block-size (bug 1117983).
Direction-independant equivalents of margin-top, margin-right, margin-bottom and margin-left: margin-block-start, margin-block-end, margin-inline-start and margin-inline-end (bug 649142).
Direction-independant equivalents of padding-top, padding-right, padding-bottom and padding-left: padding-block-start, padding-block-end, padding-inline-start and padding-inline-end (bug 649142).
Direction-independant equivalents of border-top, border-right, border-bottom and border-left and their longhands for width, style and color: border-block-start, border-block-start-width, border-block-start-style, border-block-start-color, border-block-end, border-block-end-width, border-block-end-style, border-block-end-color, border-inline-start, border-inline-start-width, border-inline-start-style, border-inline-start-color, border-inline-end, border-inline-end-width, border-inline-end-style and border-inline-end-color (bug 649142).
Direction-independant equivalents of top, right, bottom and left: offset-block-start, offset-block-end, offset-inline-start and offset-inline-end (bug 1120283).
How CSS Transitions start has been modified to match a recent change of the specifications, aiming at having an interoperable behavior between browsers (bug 960465).
HTML
Label element doesn't dispatch event to label target element if interactive content is found between the event target and the label ( makes all buttons act like first button" href="https://bugzilla.mozilla.org/show_bug.cgi?id=229925" kesrc="https://bugzilla.mozilla.org/show_bug.cgi?id=229925" style="margin: 0px; padding: 0px; color: rgb(45, 131, 213); text-decoration: none;">bug 229925).
